In the article/“Sampling for on-stream analysis and composite samples” by Christian von Alfthan, Matti Kongas; published in the publication “Recent Advances in Mineral Processing Plant Design”, SME, Oct. 1, 2009, by Deepak Malhotra, et al ISBN:978-0-87335-316-8, pp. 155-163/, there is disclosed that it is not practical to feed a full mineral processing plant process flow through an analyzer system in most cases. A representative sample is much easier to handle and analyze accurately. A primary sample is taken from a process stream in one or more steps. The flow rate of the primary sample has to be high enough to allow reliable transfer to an analyzer or composite sampler by pumped or gravity flows. Often the primary sample as to be re-sampled as the sample presentation system to the analyzer uses a lower secondary sample flow rate than the primary sample.
The present trend to use high capacity big flotation cells has increased the process flows. Thus frequently two-stage or even three-stage sampling is needed to get a suitable sample flow for analysis. The article further discloses a two-stage sampler intended for sampling gravity flows. The disclosed two-stage sampler is generally suitable for sampling of near horizontal non-pressurized pipes or launders with flows higher than 420 m3/h. The sampler comprises a first sampling unit for taking a primary sample flow from the process flow, and a second sampling unit for taking a secondary sample flow from the primary sample flow. A primary sample flow taken from the process flow has a width which is a portion of the width of the process flow. The primary sample is separated from the process flow by the first sampling unit. The separated primary sample flow is spread widthwise to a larger width. A secondary sample flow is separated by the second sampling unit from the spread primary sample flow. Finally the secondary sample flow is conducted to analysis. The sampling units are vertical cutters which extract a narrow slice from the slurry stream. In the prior art sampler the sample is re-shaped by an inclined top wall of the first sampling cutter from an initially narrow vertical cut to a broad strip with a low horizontal cross-section. This makes it possible to use a second sampling cutter to reduce the sample flow to the required size.
In the described prior art two-stage sampler, in use, both the first sampling unit and the second sampling unit are full of slurry and thus act as pressure samplers. A pressure sampler always gives a constant secondary sample flow which does not depend on the flow rate of the process flow. Therefore, the sampler is not able to give a secondary sample flow which would be proportional to the process flow.
Therefore, the problem is that the prior art two-stage sampler is not very useful to be used for composite sampling (periodic sampling). Principally, there are two reasons for slurry sampling. Firstly, the sample is analyzed in the analyzer to measure a certain instantaneous quality characteristic. The known sampler is quite suitable for that. Another reason for slurry sampling is to obtain correct data of an instantaneous volume flow in the instant of time when the instantaneous quality characteristic occurred. Therefore, there is still a need for a sampler which would be able to give a sample flow which is approximately proportional to an instantaneous flow rate of the process flow to enable correct composite sampling.
Another problem with the known two-stage sampler is that it is sensitive to changes in the process flow. If the process flow increases, backflow from the first sampling unit may occur. Also a backflow from the process flow passing through the second sampling unit may undesirably enter the second sampling unit.